As is known in the art, the compression ratio of an internal combustion engine is defined as the ratio of the cylinder volume when the piston is at bottom-dead-center (BDC) to the cylinder volume when the piston is at top-dead-center (TDC)—generally, the higher the compression ratio, the higher the thermal efficiency and fuel economy of the internal combustion engine. In spark-ignition engines, compression ratios are limited by engine knock or autoignition which tends to occur at lower engine speeds and higher engine torques. Engine knock does not typically occur at engine idle, which is a low speed, low torque condition. However, if the other factors leading to knock, such as low humidity, high ambient temperature, low octane fuel, etc. occur, the engine can knock at idle. This is further exacerbated when engine accessory loads are placed on the engine so that the torque requirements are higher than an unloaded idle condition, such accessories being air conditioning and power steering pump, as examples.
So-called variable compression ratio (VCR) internal combustion engines have been developed to allow using higher compression ratios during knock-free conditions to take advantage of the high thermal efficiency and lower compression ratios during knock prone conditions. The inventor of the present invention has recognized that the problem of idle knock could be further exacerbated if the VCR selection is in a higher compression ratio.
The inventor of the present invention has recognized that operating at a high compression ratio at idle may result in engine knock, particularly when the exacerbating conditions listed above also exist.